63A RCD for ring main?

Discussion in 'Electricians' Talk' started by robertpstubbs, Jun 14, 2018.

  1. robertpstubbs

    robertpstubbs Active Member

    Was just at a friend's for a social visit so couldn't do much there and then. There is a 2nd small CU which just contains a 63A RCD and is labelled RING MAIN. RCD is tripping and a kitchen socket, hood, and boiler controller don't work. Didn't have chance to do any tests.

    Before I go back, is there any good reason why a ring main would be wired this way?
  2. robertpstubbs

    robertpstubbs Active Member

    (I didn't even have a chance to check that the non-working stuff in the kitchen is actually on the tripping alleged RING MAIN.)
    Last edited: Jun 14, 2018
  3. dobbie

    dobbie Well-Known Member

    Are you saying that there is not an MCB in the enclosure, just an RCD main switch and the circuit fed from that.
  4. robertpstubbs

    robertpstubbs Active Member

  5. Bazza

    Bazza Well-Known Member

    Photos of this mystery 2nd CU please.
    I think you have misunderstood the setup.
    If there is an RCD, there must be an MCB protecting the circuit, unless it was installed by a numpty on a horse. Yeeehaaa!

    It could be a DP RCBO, of course. In which case the 63A value is not correct.
  6. Pollowick

    Pollowick Well-Known Member

    Come on, give the installer some credit, he would at east have tied his horse up to the gas main outside!

    There are 63A RCBOs, I believe Schneider are one supplier.
  7. Bazza

    Bazza Well-Known Member

    Yes, but not used as the CPD on a ring final - not unless its wired in 6mm!
  8. unphased

    unphased Screwfix Select

    If the main CU is without RCD protection then it could have been added on the ring circuit to provide it. However, as you've no doubt realized, its a bodge. It is probably a BS EN 61008 version, intended for use in dual RCD boards.
  9. Lectrician

    Lectrician Screwfix Select

    I would say it is an RCD external to an old non RCD consumer unit, providing retrospective RCD protection to the socket circuit, likely fed from a rewireable fuse or similar.
  10. robertpstubbs

    robertpstubbs Active Member

    May be wired off an MCB in main CU.
  11. robertpstubbs

    robertpstubbs Active Member

    Image attached.

    Attached Files:

  12. Bazza

    Bazza Well-Known Member

    Need a closer close up of the RCD and the consumer unit. But it looks like a bit of 6mm from the CU fed into the RCD with the ring bring run from the RCD. IE a lollipop circuit.

    So the RCD has needed to be added for some reason. May be because of extension or changes to that circuit required RCD protection and it was cheaper/easier to add RCD just to that circuit rather than put in a whole new CU. .

    If that RCD is tripping there’s a fault on the kitchen sockets ring final somewhere. Time to do some testing and/or elimination of suspect devices. I would start with the boiler /heating control circuits. Leaking/faulty pump or valve may be the culprit.
  13. Sparkielev

    Sparkielev Well-Known Member

    I would unplug every appliance before i started testing faulty fridge, toaster washing machine etc could be causing it
  14. MGW

    MGW Well-Known Member

    It does seem not a 63A ring final, but a ring final likely protected by a 32A MCB then a RCD rather than RCD then MCB as normal, there is nothing wrong swapping which device comes first, so next is why tripping.

    The standard procedure is always unplug where possible and switch off all fuse connection units and see if still tripping, one hopes once everything unplugged or switches off the fault is removed, if not then it is down to test gear to locate the fault.

    Once you know ring final can be powered then you know either there is a faulty appliance, or an earth to neutral fault. We hope not a earth to neutral fault as this again needs test gear.

    So next is to try a large current using item like a kettle, if this is OK then it can be then used to test for earth - neutral faults in other items, so rest of stuff is plugged back in, and after each group of item kettle tried, if plugging an item in trips the RCD great, but one has to remember the earth - neutral fault can make it seem a large power user is faulty, when in fact it is a fault with a low powered item neutral to earth.

    I used the toaster as the example of earth - neutral fault, some bread or toast is lodged between near neutral and earth which gets damp, the toaster switch is on the line, so plugged in yet switched off it still causes a neutral to earth fault, however with no or little load the neutral voltage is nearly the same as earth, so no current flows, so RCD does not trip. However when the kettle is used the neutral voltage rises compared with earth, so now current flows, so RCD trips when kettle is used, but not kettle which is faulty it is toaster. This is why you need to unplug stuff, not simply switch it off when looking for a fault.

    The other hard thing to locate is a faulty element in a frost free freezer, since only switched on during de-frost cycle, the tripping seems random.

    I bought an insulation tester for £35, I use it to test earth to neutral, not calibrated, but does a really good job finding what is causing a RCD to trip, bought it when I had a problem with caravan, by time it arrived the caravan fault had gone, and although I take it with me each time we go away, fault has never returned. Must have been some water ingress some where, but now simply can't find any fault.
  15. robertpstubbs

    robertpstubbs Active Member

    I didn't explain properly that this isn't in my friend's main house, but in his 2nd empty one. Therefore there are virtually no appliances plugged in.

    A spark did an EICR which said "faulty RCD", but didn't bother to mention that this makes the house almost unusable.

    Things worked before the EICR. Isn't the RCD ON in the image which was taken before?
  16. Bazza

    Bazza Well-Known Member

    Robert, I think we have answered your initial question
    You now have moved on to find out why the RCD is tripping. This task is not something that can be done, third hand over the Internet. You need to go on site, with the appropriate test equipment and determine the fault.
    You are going to need at least:
    A low ohm resistance tester
    An insulation resistance tester
    an RCD tester so that you can determine what the tester meant by
    Cannot see from the strange PDF image that you posted. Its too blurry.

    There may still be external influences. External sockets, for instance, and you mentioned the boiler controller, is the FCU for the heating circuit turned off?

    Let us know how you get on with your investigations.
  17. retiredsparks

    retiredsparks Well-Known Member

    Looks like the main board is fully rcd protected.?
    Why fit another ?
  18. Bazza

    Bazza Well-Known Member

    Good spot! But we don’t have details of that (need detailed photo of the cu and the cut out). This may be an old TT with a 100mA RCD main switch. So it would need a 30mA RCD for that kitchen circuit to comply with more recent regs.

    In any case, we need much more info to be able to comment, otherwise we just pi$$ing in the wind.
  19. robertpstubbs

    robertpstubbs Active Member

    What's the max size image that can be uploaded? Most of my photos are about 6MB. Printing JPG as PDF reduced size to 0.2MB with no noticeable drop in quality on my laptop.
    Admittedly photo isn't brilliant and wasn't taken for this purpose.
  20. Bazza

    Bazza Well-Known Member

    If u have jpg, you can just drag it to the reply pane. Then it becomes part of your thread. Makes it much easier to read.

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